Improvement of connectivity in Cu/OSP flip chip package using slow cure NCP

Capillary type underfill still dominates for the underfill applied to mass-produced flip chips and is dispensed after chip attachment process. Also nonconductive paste (NCP), applied before chip attachment, has been introduced for fine pitch applications. Thermal compression bonding (TCB) is adapted for this process. These pre-applied type underfills have advantages such as no need to have space to dispense and no flux cleaning process due to self fluxing of the underfill itself. However, NCP still has some problems with connectivity and void control. Ordinary NCP needs to maintain lower filler loading and flow properties and also process optimization has to be severe to achieve good connectivity and void free. Slow cure NCP is one solution to give us good connectivity and void free. This paper discusses the performance and potential of slow cure NCP for fine pitch package. Slow cure NCP achieves good connectivity even if filler entrapment happens, and void is eliminated during post cure in a pressure oven. The material properties of the underfill were characterized using viscometer, Thermo-Mechanical Analyzer (TMA), and Dynamic Mechanical Analyzer (DMA). The assembled performance was observed by Scanning Electron Microscope (SEM) and Constant-depth mode Scanning Acoustic Microscope (C-SAM). A daisy-chained test vehicle with fine pitch Cu bumps was used. A detailed study for the underfilling process flow, properties, as well as TCB results are presented.